Texas Gardener Magazine: Garden Troubleshooting

By Skip Richter, Contributing Editor

"What’s wrong with my plant?”

If you’ve gardened more than a season, you’ve probably asked this question yourself on more than one occasion.

Gardening is a wonderful challenge and it seems that every year is different as we are forever facing new pests, diseases and challenges with the weather.

How do we approach a plant problem we want to diagnose or identify? Where do we begin? The internet and gardening books have various versions of problem-solving guides that can be helpful. But they are often from different parts of the country; so particular plants, pests and diseases can vary.

Let’s start by reminding ourselves of the basics of plant physiology and how the various parts function.

Leaves

Leaves are the food-producing factories of the plant. They capture sunlight and make carbohydrates that are transported around the plant and down into the roots. So a problem in the leaves results in a progressively weaker plant due to lack of food to feed the various plant parts.

Roots

Roots take up water and nutrients that are transported through the plants to the leaves, to be the building blocks of proteins, carbohydrates and other compounds in the leaves.

When roots are damaged by physical injury, insect feeding, by nematodes or root rot, or from being waterlogged (roots need oxygen), the plant may wilt or show signs of nutrient deficiencies, even when there are adequate nutrient levels in the soil.

Stems

Stems are the highway system of the plant, transporting all these materials. So when a plant shows signs of wilting, it may be that the soil is too dry, but it could also be due to something damaging the roots, an insect feeding in the stem of the plant, or fungus or bacteria plugging the plumbing in the roots and stem.

Get close, then step back

Usually, when you encounter a problem you are bending over a plant examining it for clues.

This is the time to notice the problem closeup by examining leaves, stems, fruit and flowers, making a note of things that appear abnormal. What are you seeing? Holes in leaves? Chewed edges? Yellow leaves? Is the yellowing between the veins or in the veins? Malformed new growth? Spots on the leaves? Are the younger or older leaves affected? Leaf drop? Wilting? Is the entire plant or just a portion affected? Poor growth? Lack of healthy green color?

This closeup examination of the above-ground parts is where many gardeners stop. However there are three more steps that are often very helpful. When I am asked about a plant problem, my questions move from the closeup symptoms to these other steps.

1. After examining the plant up close, I like to step back and look at the problem in the context of the plants around the garden. Is the problem affecting a group of plants or is this an isolated instance? Is there a pattern or is it randomly affecting plants? Problems caused by insects and diseases are usually more random, while those caused by nutrition and chemicals are often widespread or affect plants in a pattern of some sort.

2. If you don’t find the cause of the plant problem from your visual examination of the plant’s above-ground parts, try examining the base of the stem and the roots. If plants are rapidly declining, pull one up to check the base of the stem and the roots for signs of rot and nematodes. When the above-ground parts of a plant are just starting to decline and you find a very rotted root system, the problem most likely started in the roots. Since roots are critical for plant health, a lot of problems can be traced to a root-related problem like rot or nematodes.

3. Finally, consider the history of the planting. What has changed recently? What has the weather been like? What have you applied to the soil or plants? Often when you get stumped, thinking through these factors can help lead you toward possible causes.

Causes of problems

There are a number of common causes to plant problems, including pests, diseases, nutrient deficiencies and excesses, over- and under-saturated soil, injury from herbicides, and environmental factors. Pests such as insects and mites cause damage by either chewing or by sucking juices out of plants.

Chewing pests include grasshoppers, beetles, caterpillars and slugs/snails. Their damage is quite evident and sometimes their frass (insect poop!) can also be seen on surfaces below the plant.

In the case of slugs and snails, there should be a slim trail left behind. When pests are present, you can capture a few and have them identified. When they are nowhere to be found, check the damaged edges to see if the feeding damage appears fresh or if the edges have dried, leaving a brown strip along the edge of the damage. Perhaps the pest is now long gone! If you don’t see pests but do see fresh damage, try going out at night with a flashlight, as some pests are nocturnal.

Holes in the leaves are usually caused by beetles or very young caterpillars. Some larger types of caterpillars prefer to feed from the edges of leaves inward. When you see a leaf that is skeletonized with the veins left behind, that is a sign of young caterpillar damage. As the pests molt and grow larger mouthparts, they are progressively able to eat more of the leaf, including the smaller veins.

Pests with piercing/sucking/rasping mouthparts cause damage that appears as small, discolored spots on the foliage, flowers or fruit, or as twisted, malformed growth on foliage or fruit.

Examples of pests with piercing/sucking/ rasping mouthparts include stinkbugs, leaf-footed bugs, fourlined plant bugs, aphids, scale, mealybugs, whiteflies and thrips.

Malformed foliage and cat-faced fruits are a sign of damage from some of these pests that occurred earlier when the foliage and fruit were developing. You may now be seeing the results as the plant parts grow abnormally, while the pests may no longer be present.

Pests that suck juices from the plant excrete those juices in a sugary substance known as honeydew. Examples of such pests include aphids, scale, mealybugs and whiteflies.

This honeydew falls onto surfaces below the pest and a black sooty mold often grows on it. Sooty mold is an indication of the presence of one of these pests. Aphid damage on young shoots and leaves can also cause them to curl up, a symptom that may be confused with damage from some weed killers or viruses.

Diseases cause a range of symptoms, including leaf spots, stem cankers, blights and (in the case of viruses) mottled and/or twisted, abnormal growth.

Fungal diseases

The three most common types of diseases are fungal, bacterial and viral.

It is risky to generalize…but…generally speaking, fungi tend to develop on foliage by forming roundish spots with tan or brown centers, sometimes surrounded by a yellow halo. Fungal leaf spots take time to develop, so they are usually on the older foliage as opposed to the new growth.

Bacterial diseases

Bacteria tend to develop more angular spots, often initially with a water-soaked appearance. Both fungi and bacteria can also live inside the “plumbing” of the plant, plugging it up and causing wilting and/or drought-like symptoms.

When only a portion of a plant wilts initially, a vascular fungal wilt is often the cause. You can often see the evidence of vascular fungal diseases by splitting the base of the stem and looking for gray to brown streaking in the interior tissues. Nematodes disrupt the normal flow of water and nutrients through the roots, so they tend to cause wilting, poor growth and nutrient-deficiency symptoms on the foliage.

Viral diseases

Viral symptoms usually appear on the younger growth, with older growth remaining normal except in prolonged cases where much of the plant becomes affected.

Some viruses cause a mottled pattern to the leaves, while others cause malformed growth in various strappy, twisting or buckling forms. Viral symptoms can look a lot like herbicide injury. While herbicide injury typically affects a large group of plants, viruses tend to affect only one plant or a random pattern of plants.

Nutrition

Nutritional deficiencies and excesses cause discoloration of the foliage and sometimes cause leaves to not reach full size.

Leaves may be yellow or purplish in color. Veins may remain green while interveinal areas turn yellow, or entire leaves or areas of a plant may start to change from a deep green color to chartreuse or yellow in color.

Various plant species show nutrient deficiencies in somewhat different ways.

One key way to diagnose nutrient deficiencies is to look at what part of the plant is affected. Some nutrients are mobile in the plant. Thus a plant can move them from older leaves to support new growth. Mobile elements include nitrogen, phosphorus, potassium and magnesium. A deficiency of these elements appears most pronounced on a plant’s older foliage.

Immobile elements cannot be moved within the plant once they become part of the plant tissue. Therefore, with immobile elements the deficiency appears primarily in the new growth. Immobile elements include calcium, sulfur, iron, zinc, boron, copper and manganese.

There are many helpful pictorial resources online for learning what a deficiency of a particular element will typically look like. Just remember that a deficiency of an element may look different from one species to the next.

Soil extremes

Soil moisture extremes, both too wet and too dry, are detrimental to plants, even to the point of being fatal.

Most folks are familiar with drought symptoms: wilting, browning of leaf tips and margins, and premature shedding of foliage.

Herbicides

Herbicides are designed to kill plants. That’s not news.

But an herbicide doesn’t typically distinguish between what you consider a weed or a desirable plant! When misused, they can do significant damage to our gardens and landscapes.

Herbicide damage symptoms are primarily either a bleaching or loss of color in the foliage (similar to some nutrient deficiencies) or malformation of foliage and young shoots (often similar to some viral infections). Herbicides common to turf products can drift onto garden plants or be washed down into the root system of trees and shrubs, causing damage.

Atrazine-type products cause yellowing of foliage, while hormone-type products like 2,4-D cause twisting of the foliage. Some pre-emergent products can cause “clubbing” of St. Augustine grass roots, resulting in a gradual decline of the plant.

Gardeners seldom intentionally apply a weed killer to their garden plants. Sometimes you have to play detective to figure out how an herbicide could have been applied to your plants.

If you use manures in your garden, it is always a good idea to make sure they are well decomposed. It would also be useful to know if the cattle grazed on a pasture treated with some selected brush control products. Some of these compounds are persistent enough to go through the cow and still be present in the manure in quantities high enough to damage sensitive plants, such as tomatoes and green beans.

I have seen damage to vegetables and fruit trees from sprayers that were previously used for herbicides now being used for insecticides or fungicides.

Some products, like hormone-type weed killers, are very difficult to wash out of a sprayer. One gardener used an herbicide and then washed out his sprayer but forgot to fill it with water and wash out the spray wand thoroughly. Then later when he used the sprayer for a pesticide, the first few plants got a strong dose of weed killer! It is best to label sprayers as being for weed killers only and use other sprayers for insecticides and fungicides.

Environment

In addition to the above causes of plant problems, there are environmental causes. These can include cold damage, sunburn and damage from pollutants in the air.

Sunburn occurs most commonly when plants are moved from low light into direct sun, or when a species is not adapted to full sun. The parts of the leaves most exposed to the sun show the most prominent burn symptoms.

Cold damage can kill leaves and shoots outright or, in minor cases, may only damage the exterior of the plant or the thinner areas of the leaf in-between veins.

Pollutants such as sulfur dioxide and ozone can cause various symptoms on sensitive plants, a common symptom being tan/brown stippling on the leaves.

Resources for help

Identifying pest, disease, nutritional and other problems can be straightforward at times but is often tricky. The symptoms I’ve mentioned above are general guides, but there are exceptions to almost every rule!

Before you reach for a spray to manage a suspected pest or disease, make sure you have identified the situation correctly. For more help in diagnosing and identifying problems, contact the Texas A&M AgriLife Extension. They offer free assistance in getting to the bottom of plant problems.

Local garden centers with trained professionals can also be a resource to help you.